Magnetic resonance tomography, also termed nuclear magnetic resonance tomography, involves the technique, which has become wide spread in the interim, for obtaining images of the body interior of a, usually living, examination object. In order to obtain an image with the aid of this method, it is firstly necessary to expose the body or the body part of the patient or test subject that is to be examined to a static basic magnetic field that is generated by a basic field magnet of the magnetic resonance system. During the acquisition of the magnetic resonance images, rapidly switched gradient fields that are generated by so called gradient coils are superposed on this basic magnetic field for the purpose of location coding.
Moreover, radiofrequency antennas are used to irradiate radiofrequency pulses of a defined field strength into the examination object. The nuclear spins of the atoms in the examination object are excited by way of these radiofrequency pulses in such a way that they are deflected from their equilibrium position parallel to the basic magnetic field by a so called “excitation flip angle”. The nuclear spins then precess about the direction of the basic magnetic field.
The magnetic resonance signals generated thereby are picked up by radiofrequency receiving antennas. Finally, the magnetic resonance images of the examination object are prepared on the basis of the received magnetic resonance signals. Each image pixel in the magnetic resonance image is in this case assigned to a small body volume, a so called “voxel”, and each brightness or intensity value of the image pixels is combined with the signal amplitude, received from this voxel, of the magnetic resonance signal.
It is clear that there is a need for the presence of the most homogeneous fields possible in this method in order to obtain suitable images. Thus, for example, field inhomogeneities of the gradient magnetic fields mean that an object is shown in a two-dimensional sectional image in a distorted fashion. If the inhomogeneities of the gradient magnetic field are known, a specific algorithm can be used to carry out a distortion correction in order to generate from such an original measured image a distortion-corrected image that correctly reproduces the proportions of the object. The mathematical formulation of such a two-dimensional distortion and of the possible distortion correction is described, for example, in the article “Simulation of the Influence of Magnetic Field Inhomogeneity and Distortion Correction in MR Imaging” by Ján Wei{hacek over (s)} and Lúbos Budinský in Magnetic Resonance Imaging, vol. 8, 1990.
In order, furthermore, to keep the inhomogeneities in the basic magnetic field as slight as possible when undertaking magnetic resonance measurement, a switch is being made more and more to using a so called isocenter scanning method for measurement. In this case, all layers as near as possible to the isocenter of the magnetic resonance unit or the basic magnet are measured by appropriately displacing the table on which the patient or test subject lies during the measurement. The result of this is that large image fields such as, for example, the spinal column cannot be measured all at once. Such a measurement must then be decomposed into a number of measuring steps, a measurement being carried out station by station at the isocenter of the magnetic resonance magnet. In order then to be able to reassemble the individual images generated in this case to form an overall image, it is necessary for them to be subjected to two-dimensional distortion correction, something which can be performed in the known way.
In the routine operation, a very large portion of the patient is in the mean time being examined using such an isocenter scanning method, the images always being subjected to distortion correction as described. In most cases, however, a spectroscopy is then further arranged by a doctor responsible after looking through the images. Thus, for example, in the case of a so called “single voxel spectroscopy” a number of voxels are selected in a defined area in the sectional image of the patient/test subject, and there is then generated for these a frequency spectrum with the aid of which metabolites can be identified. The zone in which this spectroscopy is to be carried out is defined as a rule in this case directly in the magnetic resonance images present with the aid of a graphics user surface.
If, for this purpose, the distortion-corrected images are used the problem occurs the subsequent measurement may be carried out at the wrong location. This may be explained with the aid of FIG. 1. There, the frequency f with which a specific location x is selected in the event of the emission of the corresponding radiofrequency pulses is plotted against the pertinent location x. In an ideal, that is to say completely homogeneous field, which corresponds to the ideal line depicted (dotted line), a real location x0 in the examination object is also illustrated at the location x0 in the image. This ideal behavior also indicates a distortion-corrected image. If the graphics user interface were now to be used for the spectroscopy to select exactly this point x0 in a distortion-corrected image, the measurement would however, operate with the frequency f2, since, after all, field inhomogeneities actually are present in reality. This would have the effect that finally, during the spectroscopy measurement the wrong location x2 would be excited in the examination object instead of the correct location x0, since the frequency f2 corresponds to the location x2 in accordance with the relationships actually present (continuous line). Thus, the distorted original measured image is required in order to select the correct location for a subsequent spectroscopy measurement.
In this original measured image, a location x0 is assigned a frequency f0 that is then displayed at the point x1 in the distortion-corrected image. It is then possible to select in this image the anatomy of the location x0 at the pixel x1 which contains the information relating to the location x0. The measurement is duly carried out thereupon with the frequency f0 such that measurement is performed correspondingly in the examination object at the location x0. That is to say, when selecting is specific anatomy at the pixel x1 in the distorted original measured image this is then also actually measured even if this anatomy lies at the location x0 in the real examination object.
This gives rise to the problem that, on the one hand, distortion-corrected images are required in order actually to be able to assemble images in an isocenter scanning method and, on the other hand, undistorted images are needed in order to be able to plan and correctly control subsequent measurements. It is certainly true that it would be possible for each magnetic resonance measurement with the aid of an isocenter scanning method also to store the distorted original images in the database in addition to the distortion-corrected images.
However, this is not expedient, since then the data volume to be stored is unnecessarily increased. It is to be taken into account here that, after all, only a very small portion of the images initially generated are actually used for later precise planning and control of a subsequent measurement. Since, consequently, virtually twice as many images than are actually required must be handled in the image calculation, transferred and input into the database, this necessarily results in the fact that the performance of each image calculation is worsened.
This leads overall to the fact that the examination time per patient is lengthened and the patient throughput is reduced. In addition, it can be that a few examination sequences can no longer be carried out at all, such as, for example, spectroscopies with contrast agents, since the period for preparing the first images and the further planning and determination of the location for the subsequent spectroscopy measurements lasts so long that the contrast agent is already washed out.
As an alternative it would be possible to repeat a part of the measurement before planning a spectroscopy, in order to obtain the required localization images not corrected for distortion. However, this is likewise not acceptable, since this, too, also leads to a substantial time loss and is attended by additional burdens for the patient. Here, as well, it can happen in the case of a contrast agent measurement that the actual spectroscopy measurement is no longer possible since the contrast agent has already been washed out, as conditioned by the time loss for the measurement of the additional images for planning this spectroscopy.
The outcome of this set of problems in reality was that it has so far not actually been possible to apply any isocenter scanning methods when there is a subsequent spectroscopy measurement to be carried out correctly.